In a sine wave drive system (a 180-degree energizing system) that has been increasingly adopted in application examples as a synchronous motor (permanent magnet synchronous motor) drive system, a rotor position (a rotation position of a rotor) is detected in a sensorless manner, and control for effecting appropriate energization of a stator coil is executed. As such a motor control device having a function of detecting a rotor position in a sensorless manner, a motor control device disclosed in Patent Document 1 is proposed. The motor control device in Patent Document 1 enables detection of a rotor position θm with a constant accuracy and a low processing load in a normal operation mode (positional detection operation) of the synchronous motor.
The synchronous motor has a startup mode (forced commutation operation) as a preliminary stage to a normal operation mode, and in this startup mode, since an estimated error of a rotor position θm representing an angle of d axis in a rotor coordinate system (an absolute position of the rotor) relative to α axis in a stator coordinate system becomes high, a position detection operation based on the rotor position θm is not performed. Thus, in the startup mode, the synchronous motor is started by a motor control device disclosed in Patent Document 2. Upon receiving an operation instruction including a target rotational velocity (a target number of revolutions) in a motor-stopped state, the motor control device in Patent Document 2 starts driving with an applied voltage and an applied voltage phase that are set by a startup voltage setting unit and a startup phase setting unit, whereby a rotational velocity is gradually increased with constant acceleration. When the rotational velocity reaches a predetermined value lower than a target rotational velocity, it is deemed that the startup is completed, and control for transition to the normal operation mode is executed. In this configuration, no computing device having a high processing capacity is necessary, and end of startup can be simplified and secured.